Method and device for the production of superheated steam

ABSTRACT

The invention relates to a method and an apparatus for the generation of superheated steam. According to the invention, essentially saturated or wet steam is generated in a main vessel in which superheating is technically not possible or only restrictedly possible and which is superheated in an auxiliary plant whereby the superheater of the auxiliary plant is controlled dependent upon the steam production of the main plant.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US national phase of PCT applicationPCT/DE2006/000885, filed 23 May 2006, published 8 Feb. 2007 asWO2007/014538, and claiming the priority of German patent application102005036792.5 itself filed 2 Aug. 2005, whose entire disclosures areherewith incorporated by reference.

FIELD OF THE INVENTION

The present invention relates in particular to substitute fuelutilization and waste incinerator plants. Substitute fuels areunderstood to mean all fuels whose flue gas has components that arecorrosive for the boiler and downstream plant components and/or thattend toward slagging. These are, for example, compounds containingchlorine and sulphur and/or low-melting-point ash that can have, forexample, a high alkali content.

BACKGROUND OF THE INVENTION

During the operation of waste incinerators the safe and reliabledisposal of refuse and hazardous waste stands above all in theforeground. At first, attempts have been made to limit the emission ofhazardous materials to avoid environmental damage. The increasingworld-wide scarcity of energy reserves has finally led to considerationbeing given to utilizing the calorific value of incinerator feedstock aswell as to optimizing the energy efficiency of the processes.

Most substitute fuel incinerators were hitherto equipped with boilersfor making medium pressure steam (up to 60-bar). The need to limitpressure comes from the high-temperature corrosion increasingly takingplace at steam temperatures above 370° C. to 400° C. with the materialsemployed. High-temperature corrosion can result in the steamsuperheaters having to be replaced after a short operation time of 3 to12 months. So that the steam produced can also be sufficientlysuperheated the steam pressure must be limited to a maximum of 40 to 60bar in such plants.

DE 19 15 852 [GB 1 260 131] describes a method in which a waste boilerproduces saturated steam that is further superheated in a main boilerfired by fossil fuel together with the saturated steam of the mainboiler that is also equipped with a vaporizer. Depending only upon steampressure and steam temperature only ca. 15% to 40% of the combustionthermal output of the main boiler is required for the simplesuperheating of saturated steam from a substitute fuel plant. However,in the document cited a complete power plant with vaporizer is providedfor superheating the saturated steam produced in the substitute fuelplant, a consequence of which is that the superheating plant is notconnected as auxiliary plant to increase the energy efficiency, but as amain plant, which itself must be considerably larger than the substitutefuel plant.

A method is described in EP 0 593 999 A1 in which a vaporizer fitted inthe boiler of an incinerator is charged with feed water by means ofwhich high pressure steam is generated as saturated steam or wet steamthat is passed out of the boiler and superheated in an externalsuperheater and then transferred to a high pressure steam turbine forpower generation. However, the method of steam superheating and the fuelused are not described in this document. In particular, the problem ofthe relationship between evaporator surfaces in the combustion plant andthe superheater surfaces that serve as external superheater is notdiscussed.

From the publication “Studie zum Energiepotential von KVA in derSchweiz, Baudirektion Kanton Zürich, AWEL, Amt für Abfall, Wasser,Energie and Luft” of June 2005, the waste incinerator HR-AVI Amsterdam,Netherlands, currently under construction is described. When operationalthis plant will have a net electrical efficiency of 30%. The efficiencywill be obtained by employing different measures for increasing theefficiency of the boiler and the measures detailed in the following forincreasing the turbine thermal efficiency compared to waste incineratorscurrently operated:

-   -   reduction of the condensation pressure,    -   superheating of live steam to 440° C.,    -   live steam pressure of 130 bar,    -   reheating with live steam as is common practice in nuclear power        plants,    -   multistage condensate preheating.

The investment costs for the efficiency-optimized Project HR-AVI with30% net electrical efficiency are ca. 20%-30% above the costs ofconventional waste incinerators with 22%-26% net electrical efficiency.Owing to the optimization a very considerable increase in superheatercorrosion is expected so that the corresponding crane installation forrapid exchange of the superheater bundle as an expendable part has beentaken into consideration. The increased live steam temperature and thereheating have provided the greatest part of the measures for theincrease in efficiency opposite conventional incinerator plants, thatare, however, utilized fully with the measures specified in thispublication.

For technical reasons nuclear power plants with moderator water aspressurized water or boiling water reactors cannot carry out anysignificant live steam superheating with the energy from nuclearfission. Only reheating with live steam can be carried out. However,with entire steam generation plants as external superheaters for nuclearpower plants the energy fraction remaining for external superheating ofsaturated steam is so low that complete large scale power plants ofdimensions comparable to the nuclear plant itself would be necessary.

OBJECT OF THE INVENTION

The object of the present invention is to provide efficient superheatingof saturated steam from substitute fuel incinerators or nuclear powerplants with the object of increasing electrical efficiency.

SUMMARY OF THE INVENTION

According to the invention the saturated or wet steam taken from a firststage, the main plant, is transferred to a second stage, the auxiliaryplant, and there superheated, the steam superheating being controlleddepending on the steam production of the first stage. By separation ofthe production of the saturated steam with high pressure, mainly in themain plant, and superheating of the saturated steam that is mainlycarried out in the auxiliary plant a series of advantageous over thestate of the art are achieved. In the first stage preferably waste,biomass or substitute fuel are incinerated when water is evaporated bythe heat produced. The auxiliary plant is, according to the invention,preferably operated with a fuel from which flue gas is produced withonly low corrosion and slagging potential.

According to an embodiment of the invention the auxiliary plant isoperated as circulating fluidized bed combustion (ZWS) with fluidizedbed heat exchanger (FBK). The combination of the two combustion plantswith different fuels takes place through a circulating medium,preferably water. According to the invention the main and auxiliaryplants are connected with one another by a water-steam circulation. Thisthermal coupling between the main plant as evaporator and the auxiliaryplant as external superheater has the advantage over the hitherto knownmethods that the auxiliary plant serving as superheater can be operatedwithout significant evaporator fraction and the investment costs andfuel costs for the higher-quality fuel of the auxiliary plant areminimized in comparison to the main plant. The coupling of the twoplants according to the invention has furthermore the advantage thatload fluctuations in steam generation caused by the normally poorlymeterable heterogeneous fuel of the main plant can be regulated throughthe good metering of the homogeneous fuel of the auxiliary plant so thatthe inlet temperature of the live steam and of the superheated steam tothe steam turbines can be regulated according to the demands of thesteam turbines, as a result of which wear of the turbines by temperaturestress can be minimized and automated operation simplified.

Main plants, according to the meaning of the present invention, aresubstitute fuel utilization plants, waste incinerators and biomassincinerators with biomass that contain corrosive or ash meltingpoint-lowering components in the flue gas. Furthermore, the main plantscan also be nuclear power plants, pressurized water or boiling waterreactors that because of the moderator water do not allow significantsteam superheating.

The circulating fluidized bed combustion with the fluidized-bed heatexchanger essentially does not require an evaporation component.Dependent on technology the temperatures of the circulating fluidizedbed combustion are below 900° C. in all areas. According to anotherdevelopment of the invention the firebox does not operate as a cooledreactor, rather more cooling of the furnace takes place indirectly bythe recycled and cooled circulation ashes returned from thefluidized-bed heat exchanger. The fluidized-bed heat exchanger isoperated as a superheater, whereby the heat transfer in thefluidized-bed heat exchanger as a solid bed steam superheater issignificantly more efficient than in the case of conventional flue gassteam superheaters. In this way lower heat exchange sizes and investmentcosts ensue.

As is known principally from the state of the art, primary air is addedas fluidization agent to the fluidized-bed heat exchanger, through whichthe risk of corrosion damage is correspondingly reduced, even with thelow corrosive flue gases of the auxiliary plant. By the input offluidization air in the region of the ash input into the fluidized-bedheat exchanger the ash is cooled so far that the temperatures of thesuperheater in the fluidized-bed heat exchanger can be adjusted to thematerial properties. In the highly efficient fluidized-bed heatexchanger as superheater and reheater ca. 60% to 85% of the superheatingenergy required for superheating and reheating of the saturated steamcan be supplied, depending on the fuel of the auxiliary plant. Lignite,coal, natural gas or oil can be used as fuel for the auxiliary plant, aswell as other fuels with low corrosion and slagging potential as long asadequate homogeneity is available.

The flue gas from the circulating fluidized bed combustion enters thewaste heat superheater at temperatures of 850° C. to 900° C. Dependingon the fuel and the choice of materials of the superheater bundle,reheaters can preferably be used at these temperatures. Provided thetemperature before superheating has to be lowered to <800° C., thistakes place by flue gas recirculation, upstream economizers orevaporators. With good fuel and appropriate selection of material thisis not necessary with reheaters at pressures of 15 to 40 bar directlyafter the high pressure part of the turbines. Through the absence of theeconomizer, which is located in the main plant, an air preheater isnecessary after cooling by the waste heat superheater in order to coolthe flue gases to temperatures below 200° C. The preheated primary airenters the circulating fluidized bed combustion and thus raises theadiabatic combustion temperature, as a result of which the heat fractionof the fluidized-bed heat exchanger to be transferred is increased.

According to a further embodiment of the invention, two fluidized-bedheat exchangers are used in the auxiliary plant, of which one is usedfor superheating, and the other for feed water preheating andevaporation in the case of a failure of the main plant, as start-upboiler and/or regulating variable

The flue gas from the auxiliary plant is preferably purified together atleast in part with the flue gas from the main plant and dischargedthrough a common chimney.

As mentioned above, when carrying out the method an apparatus is usedwith which the saturated or wet steam generated in a main plant is fedinto a steam generator drum and from there transferred into a separatecontrollable auxiliary plant that has a device for circulating fluidizedbed combustion with fluidized bed cooling. In addition to live steamsuperheating, the auxiliary plant is provide with a further waste heatsuperheater for reheating, whereby the superheater(s) of the auxiliaryplant is/are connected with a turbine for power generation.

BRIEF DESCRIPTION OF THE DRAWING

Further embodiments of the invention and the advantages thus achievableare explained with the diagrammatic sole FIGURE of the drawing thatschematically represents the construction of the apparatus.

SPECIFIC DESCRIPTION

As can be seen from the drawing, different respective fuels 1 and 2 areused in the main plant and in the auxiliary plant. The coupling of thetwo plants takes place through a drum for steam production into whichsaturated steam produced in the main plant is transferred and from whichit is fed into the auxiliary plant where it is superheated. Thesaturated steam that collects in the drum is produced in the vessel orboiler of the main plant that is fired with the fuel 1. The requiredfeed water is preheated in the economizer through the flue gas producedand transferred into the steam drum. The flue gas from the boiler thenpasses through the first stage of a flue gas purification plant (RRA),which can consist of, for example, a spray scrubber with attached limeslaking plant

The flue gas is transferred from the first stage of the flue gaspurification plant RRA into the second stage of the flue gaspurification plant RRA whereby, according to the diagram, the flue gasof the auxiliary plant is added and purified in the second stage. Inthis stage additives for example for flue gas purification can be added,for example Ca(OH)2/HOK, and particles are removed from the flue gas.After the second stage the residual heat and in part condensation heatof the flue gas is used in a condensate preheater before the flue gas isreleased into the atmosphere through a chimney 3. The flue gas can alsobe further cooled in a primary air preheater for the main plant and theefficiency of the boiler increased. The resulting condensate water canbe used for flue gas purification and/or as inflow for feed watertreatment.

The wet steam or saturated steam is fed from the drum into the auxiliaryplant in which fuel 2 is combusted in a circulating fluidized bed (ZWS).The heat formed is transferred to the saturated steam from the mainplant by the fluidized-bed heat exchanger (FBK-superheater) and thewaste heat superheater. The steam thus superheated is fed to the turbinefor power generation. The condensate is returned to the feed water pumpthrough the above-mentioned preheater. In order to be able to treat theflue gas from the auxiliary plant in the second stage the flue gas mustbe cooled, for which an air preheater (LUVO) is used, which preheats thecombustion air of the fluidized-bed combuster ZWS.

1. A system for making superheated steam, the system comprising: a mainplant having a drum for making saturated high-pressure steam; anauxiliary plant; means for transferring the saturated high-pressuresteam to an auxiliary plant; and means in the auxiliary plant forsuperheating the saturated or wet steam in the auxiliary plant bycirculating fluidized-bed combustion or gasification with heat transferwith solid particles of the fluidized bed; and two fluidized-bed heatexchangers of which one is used for superheating and the other forevaporation including feed water preheating in the case of failure ofthe main plant, as a start-up boiler and regulating variable.
 2. Thesystem according to claim 1, further comprising a turbine connected tothe fluidized-bed and means for superheating of the auxiliary plant. 3.A method of making superheated steam, the method comprising the steps ofsequentially: making saturated high-pressure steam in a main plant;transferring the saturated high-pressure steam to an auxiliary plant;superheating the saturated or wet steam in the auxiliary plant bycirculating fluidized-bed combustion or gasification with heat transferwith solid particles of the fluidized bed and by low-pressure heatexchange with flue gas in a waste-heat superheater; and further coolingthe flue gases in the auxiliary plant by an air preheater.
 4. A methodof making superheated steam, the method comprising the steps ofsequentially: making saturated high-pressure steam in a main plant;transferring the saturated high-pressure steam to an auxiliary plant;superheating the saturated or wet steam in the auxiliary plant bycirculating fluidized-bed combustion or gasification with heat transferwith solid particles of the fluidized bed and by low-pressure heatexchange with flue gas in a waste-heat superheater; cooling the flue gasto the water dew point by condensate preheating to produce condensatewater; and using the resultant condensate water for flue gas cleaning oras input for feed water treatment.
 5. A method for making superheatedsteam, the method comprising the steps of sequentially: incineratingwaste biomass or substitute fuel in a main plant such that heat isreleased to heat and evaporate water and make saturated high-pressuresteam in the main plant; transferring the saturated high-pressure steamto an auxiliary plant; and combusting particles of coal in a fluidizedbed in the auxiliary plant to the saturated or wet steam in theauxiliary plant by circulating fluidized-bed combustion or gasificationwith heat transfer with solid particles of the fluidized bed and bylow-pressure heat exchange with flue gas in a waste-heat superheater. 6.The method according to claim 5 wherein the main and auxiliary plantsare coupled with one another by a circulating medium.
 7. The methodaccording to claim 5 wherein the circulating bed material is cooledindirectly mainly with steam.
 8. The method according to claim 5 whereinthe flue gases from the auxiliary plant are at least in part purifiedtogether with the flue gases from the main plant and released through achimney.
 9. The method according to claim 5 wherein in addition to thesuperheating of the primary steam at least one reheating is carried outin the auxiliary plant.
 10. The method according to claim 5 wherein fluegas temperatures of at most 900° C. for superheating are produced in theauxiliary plant by flue gas circulation.